Heat-softenable toners are widely used in imaging methods such as electrostatography, wherein electrically charged toner is deposited imagewise on a dielectric or photoconductive element bearing an electrostatic latent image. Most often in such methods, the toner is then transferred to a surface of another substrate, such as, e.g., a receiver sheet comprising paper or a transparent film, where it is then fixed in place to yield the final desired toner image.
When heat-softenable toners, comprising, e.g., thermoplastic polymeric binders, are employed, the usual method of fixing the toner in place involves applying heat to the toner once it is on the receiver sheet surface to soften it and then allowing or causing the toner to cool.
One such well-known fusing method comprises passing the toner-bearing receiver sheet through the nip formed by a pair of opposing rolls, at least one of which (usually referred to as a fuser roll) is heated and contacts the toner-bearing surface of the receiver sheet in order to heat and soften the toner. The other roll (usually referred to as a pressure roll) serves to press the receiver sheet into contact with the fuser roll. In some other fusing methods, the configuration is varied and the “fuser roll” or “pressure roll” takes the form of a flat plate or belt. The description herein, while generally directed to a generally cylindrical fuser roll in combination with a generally cylindrical pressure roll, is not limited to fusing systems having members with those configurations. For that reason, the term “fuser member” is generally used herein in place of “fuser roll” and the term “pressure member” in place of “pressure roll”.
The fuser member usually comprises a rigid support covered with a resilient material, which will be referred to herein as a “base cushion layer.” The resilient base cushion layer and the amount of pressure exerted by the pressure member serve to establish the area of contact of the fuser member with the toner-bearing surface of the receiver sheet as it passes through the nip of the fuser member and pressure members. The size of this area of contact helps to establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser member. The degree of hardness (often referred to as “storage modulus”) and stability thereof, of the base cushion layer are important factors in establishing and maintaining the desired area of contact.
In some previous fusing systems, it has been advantageous to vary the pressure exerted by the pressure member against the receiver sheet and fuser member. This variation in pressure can be provided, for example in a fusing system having a pressure roll and a fuser roll, by slightly modifying the shape of the pressure roll. The variance of pressure, in the form of a gradient of pressure that changes along the direction through the nip that is parallel to the axes of the rolls, can be established, for example, by continuously varying the overall diameter of the pressure roll along the direction of its axis such that the diameter is smallest at the midpoint of the axis and largest at the ends of the axis, in order to give the pressure roll a sort of “bow tie” or “hourglass” shape. This will cause the pair of rolls to exert more pressure on the receiver sheet in the nip in the areas near the ends of the rolls than in the area about the midpoint of the rolls. This gradient of pressure helps to prevent wrinkles and cockle in the receiver sheet as it passes through the nip. Over time, however, the fuser roll begins to permanently deform to conform to the shape of the pressure roll and the gradient of pressure is reduced or lost, along with its attendant benefits. It has been found that permanent deformation (alternatively referred to as “creep”) of the base cushion layer of the fuser member is the greatest contributor to this problem.
Particulate inorganic fillers have been added to base cushion layers to improve mechanical strength and thermal conductivity. High thermal conductivity is advantageous when the fuser member is heated by an internal heater, so that the heat can be efficiently and quickly transmitted toward the outer surface of the fuser member and toward the toner on the receiver sheet it is intended to contact and fuse. High thermal conductivity is not so important when the roll is to be heated by an external heat source.
Fluoropolymers are widely used in the form of sheet, film, coatings and laminates in various fields due to their characteristic properties such as good heat resistance, good chemical resistance and good weather resistance. These materials find applications as top-layers on electrophotographic toner fuser rollers or belts to provide appropriate frictional characteristics, abrasion and wear resistance, flexibility, processability, and adhesion to a particular substrate.
Fluoroelastomers are highly fluorinated synthetic polymer with elastic properties when cross-linked. Those materials are extremely stable to oxidative, flame, and chemical attacks. Hence, fluoroelastomers are widely used in many applications, such as aerospace, military and oil well, where a stable elastomer is required for the harsh environment. Fluoroelastomers were first commercialized in the early 1950s and evolved to series of various copolymer systems. The terpolymers of vinylidene fluoride (VF2), hexafluoropropylene (HFP) and tetrafluoroethylene (TFE) were developed and commercialized first as Viton B by Du Pont in the 1960s. Fluoropolymers need crosslinking or curing process to become elastomers and the curing is about the most critical aspect to provide the superior elastic properties. There are several curing systems developed for fluoroelastomers, such as radiation, peroxide, dithiol, diamine and aromatic hydroxy compounds.
Amine and amine derivatives used to be the most common curatives for the fluoropolymer materials. Metal oxide was always used together with amines for the curing process. It was reported that fluoropolymers are cured by amines in a three-step process. The first step is the elimination of hydrogen fluoride by bases to form double bonds. The second step involves nucleophilic addition of amines to the double bonds and the last step is the elimination of hydrogen fluoride again to form imines in the crosslinking structures. However, it was found that amines and polyamines, especially those aliphatic compounds, are too reactive for the curing process. In addition, the formed imine structures are vulnerable to different kinds of cleavage reactions. Due to those deficiencies the use of amine curatives becomes less and less popular. Instead a type of aromatic hydroxy compound curatives was developed and widely used curing agents for the fluoroelastomer applications. The commonly used aromatic hydroxy compound for the fluoroelastomer curing process is 4,4′-(hexafluoroisopropylidene)diphenol (bisphenol AF). One common curing agent conjunction with bisphenol AF is Curative 50™ from Du Pont, which is claimed to be a mixture of bisphenol AF and benzyltriphenylphosphonium salt complex.
The crosslinking mechanism of fluoroelastomer by bisphenol AF was reported to go through fluoride elimination and nucleophilic addition steps as well (P. K. Venkateswarlu, R. E.; Guenthner, R. A., Polymer Preprints 31 (1), 360 (1990)). However, unlike amine compounds, bisphenol AF alone is hardly active toward either elimination or nucleophilic addition reaction. The real active species in the crosslinking reaction is claimed to be the bisphenol AF phosphonium salt, which is a critical intermediate generated by the phosphonium component in the mixture.
U.S. Pat. No. 6,696,158 discloses a fuser member having a support and overlaid on the support a layer including a fluorocarbon thermoplastic random copolymer, a curing agent, a particulate filler containing zinc oxide, and an aminosiloxane. The fuser member disclosed in this patent has improved toner release and mechanical strength.
However, the coating composition used to manufacture the fuser member in U.S. Pat. No. 6,696,158 has a limited lifetime.